Let’s look at important things here –
Mercury - Mercury represents communication skills, marketing, calculative ability, younger siblings, hobbies, skills with hands, intelligence, quick decision making, logical thinking, youthfulness etc.
Purva Ashadha Nakshatra - It is a nakshatra of higher education, analysis and research. It is also nakshatra of giving care to others. For more information on Purva Ashadha Nakshatra, please read this - https://www.astrosaxena.com/panaksh .
Sagittarius - As Purva Ashadha is part of Sagittarius sign, Sagittarius and things represented by Sagittarius are important here. Sagittarius is the 9th sign of Zodiac Belt, hence it represents many things related with the 9th house of horoscope like Religion, Knowledge, Wisdom, Higher Learnings, Gurus, Teachings from your Father etc. Besides it also represents Coaching, Universities, Professors and Preachers, Higher Wisdom, Written Law of the Land etc. Sagittarius has 2 and half nakshatra which are Mula, Purva-Ashadha and Uttara-Ashadha. Sagittarius' lord is Jupiter.
Venus - As Venus rules Purva Ashadha, Venus's position and dignity is important to know about the overall functioning of any planet in Purva Ashadha.
Jupiter - As Jupiter rules Sagittarius, their position and dignity is important to know about the overall functioning of any planet in Purva Ashadha/Sagittarius.
Houses Involved – Also, we can’t afford to forget the houses involved as a planet can be ruling different houses and sitting in different house for every person as per ascendant and planet’s position. To know overall affect, we have to consider the houses involved as those areas of life are being impacted.
Interpretation of Mercury in Purva Ashadha Nakshatra – Now, Mercury is in enemy sign ruled by Jupiter but it is in friendly nakshatra ruled by Venus. So, it again gets into an average position. Somehow, Mercury in Sagittarius, in any nakshatra, will show a teacher, counselor or someone who is into sharing his knowledge with others. As Purva Ashadha is nakshatra of higher education, these people can be inclined towards getting higher education or sharing their higher education. They can be Professors at University level. Even otherwise, they will communicate like a very learned person. They can be very good at analyzing or rationalizing in matters of knowledge and education. As Purva Ashadha is ruled by Venus, they can be interested in financial side of education. They can be equally interested in creative side of business or education. Their creative interests can again be related with knowledge and higher education.
In relationship matters, it can be a difficult position as Mercury is in enemy sign of Jupiter. It will lead to frustration in relationship matters but it is not as troublesome as Mercury in Moola Nakshatra. Here, partners can have disagreements and frustrations but things can still work out if there is no other problematic position for relationship matters.
Role of Venus – Venus placement is important to guide Mercury in right direction, being Purva Ashadha's lord.
Role of Jupiter - Also, Jupiter placement is important to guide Mercury in right direction, being Sagittarius lord.
Other Factors – And of course, we need to consider conjunction, aspects, house lordship, house position and dashas etc to know the exact results of Mercury in Purva Ashadha. So, we always need to take a holistic view of chart rather than just 1 planetary position.
Conclusion – So, this is how I see Mercury in Purva Ashadha can work in a chart.